The aim of this study was to determine if once daily insulin detemir reverses decline in weight and lung function in patients with cystic fibrosis (CF). 12 patients with early insulin deficiency and six with CF related diabetes (aged 7.2-18.1 years) were treated for a median of 0.8 years. Changes in weight and lung function following treatment were compared to pretreatment changes. Before treatment, the change in weight SD score (ΔWtSDS), percentage of predicted forced expiratory volume in 1 s (Δ%FEV(1)) and percentage of predicted forced vital capacity (Δ%FVC) declined in the whole study population (-0.45±0.38, -7.9±12.8%, -5.8±14.3%) and in the subgroup with early insulin deficiency (-0.41±0.43, -9.8±9.3%, -6.8±10.3%). Following treatment with insulin ΔWtSDS, Δ%FEV(1) and Δ%FVC significantly improved in the whole study population (+0.18±0.29 SDS, p=0.0001; +3.7±10.6%, p=0.007; +5.2±12.7%, p=0.013) and in patients with early insulin deficiency (+0.22±0.31 SDS, p=0.003; +5.3±11.5%, p=0.004; +5.8±13.4%, p=0.024). Randomised controlled trials are now needed.